Control of ph kinetics in an electrolytic cell having an acid-intolerant alkali-conductive membrane

ABSTRACT

Systems and methods for recovering chlorine gas or an alkali metal from an electrolytic cell having an acid-intolerant, alkali-ion-selective membrane are disclosed. In some cases, the cell has an anolyte compartment and a catholyte compartment with an acid-intolerant, alkali-ion selective membrane separating the two. While a cathode is disposed within a catholyte solution in the catholyte compartment, a chlorine-gas-evolving anode is typically disposed within an aqueous alkali-chloride solution in the anolyte compartment. As current passes between the anode and cathode, chlorine ions in the anolyte solution can be oxidized to form chlorine gas. In some cases, the cell is configured so the chlorine gas is rapidly removed from the cell to inhibit a chemical reaction between the chlorine gas and the anolyte solution. In some cases, a vacuum or a heating system is used to increase the rate at which chlorine gas exits the cell. Other implementations are also described.

CROSS-REFERENCE TO RELATED APPLICATION

This application claims priority to, and the benefit of, U.S. Provisional Application No. 61/431,356, filed Jan. 10, 2011, entitled “Recovery of Useful Chemicals from Alkyl Sulfonate Stream,” the entire disclosure of which is hereby incorporated by reference.

FIELD OF THE INVENTION

The present invention relates in general to electrochemical cells. More particularly, the present invention relates to systems and methods for operating an electrochemical cell comprising an acid-intolerant, alkali-ion-selective membrane (such as a NaSICON-type or a LiSICON-type membrane).

BACKGROUND OF THE INVENTION

Electrolytic cells comprising ceramic membranes that selectively transport ions are used in a variety of processes. By having an ion-selective membrane in an electrolytic cell, certain ions are allowed to pass between the cell's anolyte compartment and catholyte compartment while other chemicals are maintained in their original compartments. Thus, through the use of an ion-selective membrane, an electrolytic cell can be engineered to be more efficient and to produce different chemical reactions than would otherwise occur without the membrane.

These ion-selective membranes can be selective to either anions or cations. Moreover, some cation-selective membranes are capable of selectively transporting alkali cations. By way of example, NaSICON-type (Na Super Ion CONducting) membranes selectively transport sodium cations, while LiSICON-type (Li Super Ion CONducting) and KSICON-type (K Super Ion CONducting) membranes selectively transport lithium and potassium cations, respectively.

Electrolytic cells comprising alkali-cation-selective membranes are used to produce a variety of different chemicals and to perform various chemical processes. In some cases, such electrolytic cells convert alkali salts into corresponding acids. In other cases, such electrolytic cells may also be used to separate alkali metals from mixed alkali salts. Additionally, as some electrolytic reactions occur in the anolyte compartment, such reactions can result in proton and acid formation and, thereby, result in the corresponding lowering of pH within that compartment.

Low pH anolyte solutions in electrolytic cells having an alkali-conducting ceramic membrane may cause some challenges for cell operation—especially where the cell membrane is intolerant to such low pH conditions. In one example, at a lower pH (such as a pH less than about 5) certain alkali-conducting ceramic membranes (such as NaSICON-type and LiSICON-type membranes) become less efficient or unable to transport sodium a lithium cations, respectively. Accordingly, as the electrolytic cell operates and acid is produced in the anolyte compartment, the cell becomes less efficient or even inoperable. In another example, acid produced in the anolyte compartment can actually damage the NaSICON-type or LiSICON-type membrane and thereby shorten the membrane's useful lifespan.

Thus, while electrolytic cells comprising a catholyte compartment and an anolyte compartment that are separated by a cation-conductive membrane are known, challenges still exist. Accordingly, it would be an improvement in the art to augment or even replace current electrolytic cells and associated methods with other cells or methods for using such cells.

BRIEF SUMMARY OF THE INVENTION

The present invention provides systems and methods for recovering chlorine gas or an alkali metal from an electrolytic cell having an acid-intolerant, alkali-conductive membrane (such as a NaSICON-type or LiSICON-type membrane). In some cases, the cell has an anolyte compartment and a catholyte compartment with the described membrane separating the two. While a cathode is disposed within a catholyte solution in the catholyte compartment, a chlorine-gas-evolving anode (such as a dimensionally stable anode, a ruthenium dioxide anode, etc.) is typically disposed within an aqueous alkali-chloride solution (such as an aqueous sodium-chloride (or NaCl) or an aqueous lithium-chloride (or LiCl) solution) in the anolyte compartment. Thus, as current passes between the anode and the cathode, chloride ions in the anolyte solution can be oxidized at the anode to form chlorine gas and the alkali cations (such as Ni⁺ or Li⁺) can be selectively transported through the membrane into the catholyte compartment.

As chlorine gas can react with water in the aqueous anolyte solution to form hydrochloric acid and hypochlorus acid, which in turn can lower the pH of the anolyte solution and, thereby, damage the acid-intolerant membrane, in some cases, the cell is configured so that chlorine gas exits the cell relatively quickly to inhibit a chemical reaction between the chlorine gas and the anolyte solution. In one non-limiting example showing how the cell can be configured to allow chlorine gas to exit the anolyte compartment relatively quickly, the anode is disposed horizontally within the anolyte compartment, relatively close to the surface of the anolyte solution. Accordingly, after the chlorine gas is formed, the gas may only need to travel a short distance before being released from the anolyte solution. Thus, the chlorine gas may have relatively little opportunity to react with water in the anolyte solution.

In another non-limiting example, the anode is permeable to chlorine gas so as to prevent the anode from blocking the chlorine gas' egress from the anolyte compartment. In another non-limiting example, a vacuum system is used to lower pressure in the anolyte compartment and, thereby, increase the rate at which chlorine gas exits the anolyte solution. Lowering the pressure also decreases the solubility of chlorine gas in the anolyte solution. In still another non-limiting example, a heating system is used to heat the anolyte solution to reduce the solubility of chlorine gas in the anolyte solution and, thereby, increase the rate at which the gas exits the anolyte compartment. In still another non-limiting example, the concentration of the alkali-chloride (such as NaCl or LiCl) in the anolyte solution is relatively high (e.g., between about 10 and about 40% by weight for NaCl or between about 10 and about 80% by weight for LiCl) to reduce the solubility of chlorine gas in the solution. In yet another non-limiting example, a hydrophobic membrane that is permeable to chlorine gas is placed over the anolyte compartment to allow the chlorine gas to escape from the compartment while preventing the anolyte solution from evaporating out of the anolyte compartment.

While the described systems and methods can be particularly useful for the collection of chlorine gas and sodium or lithium metal (e.g., in a compound, such as NaOH or LiOH, that is formed in the catholyte compartment as a result of the reactions that take place in the cell), the skilled artisan will recognize that the described systems and methods may be modified to produce a variety of other chemical products, including, without limitation, Na or Li metal, hydrogen gas, Na or Li containing organic compounds (e.g. sodium methylate) in the catholyte compartment, and Na or Li hypochlorite in the anolyte compartment.

These features and advantages of the present invention will become more fully apparent from the following description and appended claims, or may be learned by the practice of the invention as set forth hereinafter.

BRIEF DESCRIPTION OF THE SEVERAL DRAWINGS

In order that the manner in which the above-recited and other features and advantages of the invention are obtained and will be readily understood, a more particular description of the invention briefly described above will be rendered by reference to specific embodiments thereof that are illustrated in the appended drawings. Understanding that the drawings depict only typical embodiments of the invention and are not therefore to be considered to be limiting of its scope, the invention will be described and explained with additional specificity and detail through the use of the accompanying drawings in which:

FIG. 1 depicts a schematic diagram of a representative embodiment of an electrolytic cell comprising an acid-intolerant, alkali-ion-selective membrane;

FIG. 2 depicts a schematic diagram of a representative embodiment of the electrolytic cell, wherein an anolyte compartment is disposed above a catholyte compartment in the cell; and

FIG. 3 depicts a schematic diagram of a representative embodiment of an electrolytic system comprising a representative embodiment of the electrolytic cell, a vacuum system, and a heating system.

DETAILED DESCRIPTION OF THE INVENTION

Reference throughout this specification to “one embodiment,” “an embodiment,” or similar language means that a particular feature, structure, or characteristic described in connection with the embodiment is included in at least one embodiment of the present invention. Thus, appearances of the phrases “in one embodiment,” “in an embodiment,” and similar language throughout this specification may, but do not necessarily, all refer to the same embodiment.

Furthermore, the described features, structures, or characteristics of the invention may be combined in any suitable manner in one or more embodiments. In the following description, numerous specific details are provided, such as examples of suitable alkali-ion-selective membranes, anode materials, cathode materials, anolyte solutions, catholyte solutions, etc., to provide a thorough understanding of embodiments of the invention. One having ordinary skill in the relevant art will recognize, however, that the invention may be practiced without one or more of the specific details, or with other methods, components, materials, and so forth. In other instances, well-known structures, materials, or operations are not shown or described in detail to avoid obscuring aspects of the invention.

The present invention relates to systems and methods for recovering useful chemicals, including, but not limited to, chlorine gas and/or an alkali metal (or a compound comprising an alkali molecule) from an aqueous anolyte solution in an electrolytic cell that has an acid-intolerant, alkali-ion-selective membrane. While the electrolytic cell can comprise any suitable component, FIG. 1 shows a non-limiting embodiment in which the electrolytic cell 10 comprises an acid-intolerant, alkali-conductive membrane 15 separating an anolyte compartment 20 from a catholyte compartment 25. Additionally, FIG. 1 shows that while the anolyte compartment 20 can house an aqueous alkali-chloride anolyte solution 30 that is in contact with a chlorine-gas-evolving anode 35, the catholyte compartment 25 can house a catholyte solution 40 that is in contact with a cathode 45. FIG. 1 also shows that the cell 10 comprises a power source 50 that is capable of passing current between the anode 35 and the cathode 45.

With respect to the membrane 15, the membrane can comprise virtually any known or novel alkali-ion-selective membrane that is intolerant to acid or that otherwise functions less efficiently or is damaged when it is placed in contact with a solution having a pH below 5. In some embodiments, the membrane is also impermeable or substantially impermeable to water. In such embodiments, while the membrane is capable of selectively transporting specific alkali cations (e.g., Na⁺ or Li⁺) from the anolyte compartment 20 to the catholyte compartment 25, the membrane prevents the anolyte solution 30 and the catholyte solution 40 from mixing with each other.

Some non-limiting examples of suitable alkali-ion-conductive membranes 15 include any known or novel type of NaSICON-type membranes (including, but not limited to, NaSICON and NaSICON-type membranes produced by Ceramatec, Inc., Salt Lake City, Utah), LiSICON membranes (including, without limitation, LiSICON and LiSICON-type membranes produced by Ceramatec, Inc.), sodium beta-alumina, lithium aluminum titanium phosphate (LATP), La_(x)Li_(y)TiO_(3-z) type perovskite, Li₂O—Al₂O₃—TiO₂—P₂O₅ glass or Li₂S—P₂S₅ Thio-LiSICON type materials, and other related alkali-ion-selective ceramic membranes that are intolerant to low pHs. Indeed, in some embodiments, the alkali-ion-selective membrane comprises a NaSICON-type membrane that is capable of selectively transporting sodium ions from the anolyte compartment 20 to the catholyte compartment 25. In other embodiments, however, the alkali-ion-conductive membrane comprises a LiSICON-type membrane that is capable of selectively transporting lithium ions from the anolyte compartment to the catholyte compartment.

The anolyte solution 30 can comprise virtually any aqueous solution that allows the anode 35 to evolve chlorine gas, to drive alkali cations (e.g., Na⁺ or Li⁺) through the membrane 15, and/or to cause any other desired electrochemical reactions to occur when current passes between the anode and the cathode 45. In some embodiments, however, the anolyte comprises an aqueous alkali-chloride solution. In one non-limiting example in which the alkali-ion-selective membrane comprises a NaSICON-type material, the anolyte solution comprises an aqueous sodium chloride solution. In a second non-limiting example in which the alkali-ion-selective membrane comprises a LiSICON-type material, the anolyte solution comprises an aqueous lithium chloride solution. Additionally, while the anolyte solution can be produced in any suitable manner, including, without limitation, by mixing an alkali-chloride salt with water, in some embodiments, the anolyte solution comprises seawater, brine, a chemical waste stream, and/or any other suitable solution.

The chlorine-gas evolving anode 35 can comprise any suitable anode that is capable of preferentially evolving chlorine gas over oxygen gas as current is passed between the anode and the cathode 45. Some non-limiting examples of suitable anode materials comprise dimensionally stabilized anode (DSA) materials; metals and alloys of Ru, Ir, Co, Sn, Pd, and Pt; oxides of these metals, such as ruthenium dioxide (RuO₂), and other suitable anode materials that are capable of oxidizing chloride ions in the anolyte solution 30 to produce chlorine gas. Indeed, in some embodiments, the anode comprises DSA materials and/or ruthenium(IV) oxide. Thus, FIG. 1 shows that as current (not shown) is passed between the anode 35 and the cathode 45, chloride ions (e.g., Cl⁻ from the ionized alkali chloride (e.g., NaCl→Na⁺+Cl⁻ or LiCl→Li⁺+Cl⁻)) are oxidized to form chlorine gas (e.g., via the reaction 2Cl⁻→Cl₂+2e⁻), while the corresponding cations (e.g., Na⁺ or Li⁺) are selectively transported through the membrane 15. Accordingly, in some embodiments, the anode allows the chemicals in the anode compartment to have an overall reaction selected from 2NaCl→2Na⁺+Cl₂+2e⁻ and 2LiCl→2Li⁺+Cl₂+2e⁻.

With respect to the catholyte solution 40, it can comprise virtually any solution that allows the cell 10 to function as described (e.g., to produce chlorine gas) and that allows the cathode to cause a desired electrochemical reaction to occur in the catholyte compartment 25 when current passes between the anode 35 and cathode 45. Indeed, in some embodiments, the catholyte solution comprises an aqueous solution or a non-aqueous solution. For instance, the catholyte solution can comprise an aqueous or a non-aqueous alkali-salt solution (e.g., an alkali-chloride solution), an aqueous or a non-aqueous hydroxide solution (e.g., an alkali-hydroxide solution), an aqueous or a non-aqueous organic solution (e.g., an alcohol), and/or combinations thereof. Hydrogen gas generation typically happens at the cathode when the above mentioned catholyte solutions are used. By way of non-limiting example, where the alkali-ion-conductive membrane 15 comprises a NaSICON-type membrane, the catholyte solution can comprise an aqueous or a non-aqueous sodium chloride solution, sodium methoxide solution, or an aqueous or a non-aqueous sodium hydroxide solution. In some cases no catholyte solution is necessary and the cathode may comprise molten sodium metal. Similarly, where the alkali-ion-conductive membrane comprises a LiSICON-type membrane, the catholyte solution can comprise an aqueous or a non-aqueous lithium-chloride solution, or an aqueous or a non-aqueous lithium hydroxide solution. Accordingly, FIG. 1 shows that in some embodiments in which the catholyte solution 40 comprises an aqueous solution, hydrogen in water can be reduced to hydrogen gas (H₂) to release hydroxide ions (OH⁻), which can react with the cations (e.g., Na⁺ or Li⁺) that are transported through the membrane 15 (e.g., to form NaOH or LiOH). Accordingly, FIG. 1 shows one non-limiting embodiment of a system for recovering an alkali metal (e.g., as NaOH or LiOH) from the anolyte solution 30.

The cathode 45 can comprise any suitable material that allows the cell 10 to function as described herein and that also allows the cathode to reduce chemical species in the catholyte solution 40, or to perform any other suitable reaction. Some non-limiting examples of suitable cathode materials include nickel, titanium, stainless steel, graphite, nickel-cobalt-ferrous alloys (e.g., a KOVAR® alloy), a cermet material, platinized nickel, platinized titanium, a platinized cermet, and one or more other known or novel cathode materials. In some non-limiting embodiments, however, the cathode comprises nickel.

The cathode 45 can have any characteristic that allows it to function as intended. Indeed, in some embodiments, the cathode comprises a mesh structure, a porous structure, a micro-porous structure, or some other structure that provides the cathode with a relatively large surface area. In this regard, some embodiments of the cathode comprise a mesh structure.

With respect to the power source 50, the power source can be connected to the anode 35 and the cathode 45 to apply a voltage and current between the two electrodes to drive reactions within the electrochemical cell 10. This power source can be any known or novel power source suitable for use with the described electrochemical cell.

When chlorine gas reacts with water, it can form hydrochloric acid and hypochlorous acid (e.g., via the reaction Cl₂+H₂O→HOCl+HCl). Thus, if significant amounts of chlorine gas were able to react with the aqueous anolyte solution 30, the pH of the anolyte solution would be lowered—potentially reducing the efficiency of, or even damaging, the acid-intolerant membrane 15. To prevent chlorine gas formed in the anolyte compartment 20 from reacting with the anolyte solution, some embodiments of the cell 10 are configured in such a manner that chlorine gas exits the anolyte compartment relatively quickly. In this regard, the cell can comprise any suitable mechanical characteristic (or combination of mechanical characteristics) for allowing chlorine gas to exit the cell quickly, and/or the cell may comprise any suitable mechanism or combination of mechanisms for varying the kinetics of the anolyte solution (e.g., via changing the pressure and/or temperature within the anolyte compartment) to increase the rate at which the chlorine gas leaves the solution.

In one non-limiting embodiment, while the anode 35 can be disposed within the anolyte compartment 20 in any suitable orientation (including, without limitation) vertically, diagonally, horizontally, etc.), FIG. 2 shows an implementation in which the anode 35 is disposed substantially horizontally in the anolyte compartment 20, near the surface of the anolyte solution 30. Accordingly, when chlorine gas is formed at the anode, such gas may be able to move a relatively short distance before leaving the anolyte solution (as opposed to when the anode is disposed vertically within the anolyte solution), and thereby have a relatively lower likelihood of reacting with water as the gas exits the compartment.

Where the anode 35 is disposed substantially horizontally within the anolyte compartment 20, near the surface of the anolyte solution 30, any suitable amount of the anolyte solution can cover the anode. In some embodiments, the upper-most surface 55 (shown in FIG. 2) of the anode 35 is disposed below the surface of the anolyte solution by less than an amount selected from about 4 centimeters (cm), about 2 cm, about 1 cm, about 0.5 cm, about 0.25 cm, and about 0.1 cm. Indeed, in some embodiments, the upper-most surface of the anode is above the surface of the anolyte solution, while most of the anode is within the anolyte solution.

Where the anode 35 is disposed substantially horizontally within the cell 10, the anolyte compartment 20 and the catholyte compartment 25 can also have any orientation in the cell 10 (e.g., the alkali-ion-selective membrane 15 can have any suitable orientation in the cell) that allows the cell to function as intended. Indeed, while the anolyte 20 and catholyte 25 compartments can be disposed side-by-side (as shown in FIG. 1), FIG. 2 shows an embodiment in which the alkali-ion-selective membrane 15 runs substantially horizontally within the cell 10 so that the anolyte compartment 20 is disposed above the catholyte compartment 25. While the configuration in FIG. 2 may perform several functions, in some cases, by having the anolyte compartment 20 disposed above the catholyte compartment 25, the cell 10 is configured to rapidly release chlorine gas from the anolyte compartment (e.g., by holding the anode 35 in a substantially horizontal position, that runs substantially parallel to the cathode 45).

In another non-limiting embodiment, the anode 35 can have any suitable shape that allows chlorine gas to readily flow past it. For instance, the anode can comprise a mesh structure, a porous structure, a wire structure, an expanded metal type mesh structure, or some other single or multilayered structure that allows chlorine gas to flow through or around the anode. Indeed, in some instances, the anode comprises a perforated structure (e.g., a mesh structure).

In still another non-limiting embodiment, in order to ensure that chlorine gas exits the anolyte compartment 20 relatively quickly, the anolyte solution 30 comprises a relatively high concentration of the alkali-chloride (e.g., NaCl or LiCl), which, in turn, lowers the solubility of chlorine gas in the anolyte solution. In this regard, the concentrated anolyte solution can comprise any suitable concentration of the alkali-chloride that reduces the solubility of chlorine gas in the anolyte solution. Indeed, in some embodiments in which the concentrated anolyte solution comprises sodium chloride, the anolyte solution comprises sodium chloride at an amount as high as a concentration selected from about 20%, about 30% about 40%, and about 50%, by weight. In contrast, in some embodiments, the concentrated anolyte solution comprises sodium chloride at an amount as low as a concentration selected from about 5%, about 10%, about 15%, and about 18%, by weight. Additionally, in some embodiments, the concentrated anolyte solution comprises any suitable combination or sub-range of the aforementioned sodium chloride concentrations. In one non-limiting example, the concentrated anolyte solution comprises between about 10% and about 40% sodium chloride, by weight.

In some embodiments in which the concentrated anolyte solution 30 comprises lithium chloride, the anolyte solution comprises lithium chloride at an amount as high as a concentration selected from about 30%, about 40%, about 50%, about 60%, and about 80%, by weight. In contrast, in some embodiments, the anolyte solution comprises lithium chloride at an amount as low as a concentration selected from about 10%, about 20%, about 30%, and about 50%, by weight. Additionally, in some embodiments, the anolyte solution comprises any suitable combination or sub-range of the aforementioned lithium chloride concentrations. Indeed, in one non-limiting example, the anolyte solution comprises between about 50% and about 80% lithium chloride, by weight.

In yet another non-limiting embodiment, in order to help chlorine gas rapidly exit the anolyte solution 30, the cell 10 optionally comprises means for increasing the rate at which chlorine gas exits the anolyte solution 30. While such means can include any suitable mechanism or component that serves to increase the rate at which chlorine gas exits the anolyte compartment 20, some non-limiting examples of such means include a vacuum and/or a heating system.

In one example of the means for increasing the rate at which chlorine gas exits the anolyte solution 30, FIG. 3 shows an embodiment in which the cell 10 comprises a vacuum system 60 that lowers the pressure in the anolyte compartment 20 and acts to withdraw chlorine gas from the anolyte solution 30. In this example, the vacuum system can comprise any vacuum suitable for use with the described electrochemical cell. An applicable vacuum range is between 760 to 1×10⁻³ Torr, while a preferred range is between 100 to 1 Torr.

Where the means for increasing the rate at which chlorine gas exits the anolyte solution 30 comprises a heating system 65 (a non-limiting embodiment of which is shown in FIG. 3), the heating system can comprise any heater, temperature controller, or other mechanism that is capable of adjusting (e.g., heating) the temperature of the anolyte solution so as to increase the rate at which chlorine gas exits the cell 10 (e.g., in accordance with Henry's law). In this regard, the heating system can heat the anolyte solution to any suitable temperature that increases the rate at which chlorine gas leaves the anolyte solution that is faster than the rate at which the gas would leave the anolyte compartment at standard temperature and pressure. In some embodiments, the heating system raises and/or maintains the temperature of the anolyte solution to a temperature as high as a temperature selected from about 60° Celsius (C), about 65° C., about 70° C., about 80° C., about 90° C., and about 95° C. In some embodiments, the heating system raises and/or maintains the temperature of the anolyte compartment to a temperature as low as a temperature selected from about 35° C., about 40° C., about 45° C., about 50° C., and about 55° C. Additionally, in some embodiments, the heating system raises and/or maintains the anolyte solution at any suitable combination or sub-range of the aforementioned temperatures. Indeed, in one non-limiting example, the heating system raises and/or maintains the anolyte solution at a temperature between about 40° C. and about 65° C.

To further aid in the removal of chlorine gas from the anolyte compartment 20, FIG. 2 shows that some embodiments of the cell 10 optionally comprise a hydrophobic membrane 70 that is permeable to chlorine gas. While this membrane can perform any suitable function, in some embodiments, the membrane covers an opening of the anolyte compartment 20 so as to allow chlorine gas to escape from the anolyte compartment while preventing the anolyte solution 30 from evaporating out of the compartment (e.g., as the anolyte solution is heated).

Where the cell 10 comprises the chlorine-gas-permeable, hydrophobic membrane 70, the membrane can be made out of any suitable material, including, without limitation, polyvinylidenefluoride, polyethersulfone, nylon, and/or polytetrafluoroethylene (Teflon®) (e.g., a micro-porous polytetrafluoroethylene). Additionally, where the cell comprises the chlorine-gas-permeable, hydrophobic membrane, the membrane can have any suitable characteristic that allows it to perform its intended functions. For instance, the chlorine-gas-permeable, hydrophobic membrane can have any suitable thickness. Indeed, in some embodiments, the chlorine-permeable membrane is as thick as a thickness selected from about 5 micrometers (μm), about 6 μm, about 8 μm, and about 10 μm. In contrast, in some embodiments, the chlorine-permeable membrane is as thin as a thickness selected from about 0.1 μm, about 0.5 μm, about 2 μm, and about 4 μm. Additionally, in some embodiments, the chlorine-gas-permeable membrane can be any suitable combination or sub-range of the aforementioned thicknesses. Indeed, in one non-limiting example, the chlorine-gas-permeable membrane has a thickness between about 0.5 μm and about 8 μm.

The described systems and methods can be varied in any suitable manner. For instance, in addition to the described components, the electrochemical cell 10 may comprise any other suitable component, such as a conventional pH controlling system, a secondary cathode, a sacrificial cathode, an alkaline buffer, etc. Indeed, in one example, additional chemical ingredients are added to the cell for any suitable purpose (e.g., to modify fluid pH, to combat scaling on the electrodes, etc.). In another non-limiting example, effluents from one compartment are fed into a desired compartment (e.g., anolyte compartment 20 and/or catholyte compartment 25) at any suitable time and in any suitable amount. In still another non-limiting example, the various compartments of the electrochemical cell can also comprise one or more fluid inlets and/or outlets. In some embodiments, the fluid inlets allow specific chemicals and fluids to be added to one or more desired places within the cell. For instance, the fluid inlets may allow a chemical to be added to the anolyte compartment or the catholyte compartment. In other embodiments, the fluid inlets and outlets may allow fluids to flow through one or both compartments in the cell. In still other embodiments, these inlets and outlets are also used to interconnect one or more of the cell's compartments. By interconnecting the cell's compartments, outlet streams or effluents from one compartment may be mixed with the contents the other compartment.

The electrolytic cell 10 may function in any suitable manner. Generally, however, the described cell is provided (as discussed above) and the various chemical ingredients (e.g., the anolyte solution 30 and/or the catholyte solution 40) and/or products are added to and/or removed from the cell in any suitable manner, including, without limitation, by being batch fed and/or removed or by being continuously fed and/or removed from the cell. Additionally, as the cell functions (e.g., current is passed between the anode 35 and the cathode 45), chlorine gas is produced in the anolyte compartment 20, and that gas rapidly exits the anolyte solution 30 (e.g., as a result of the vacuum system 60, heating system 65, proximity of the anode 35 to the surface of the anolyte solution, the permeability of the anode to chlorine gas, the chlorine-permeable membrane 70, and/or any other component or characteristic of the cell that allows chlorine gas to rapidly exit the anolyte compartment). The chlorine gas that exits the anolyte solution can then be used for any suitable purpose, including, without limitation, for the production of bleach (e.g., via the combination of the chlorine with sodium hydroxide), for generation of hypochlorous acid, or for any other purpose in which chlorine gas can be useful. Additionally, after the alkali ion (e.g., Na⁺ or Li⁺) passes through the acid-intolerant, alkali-ion-selective membrane 15, the alkali ion can be used for any suitable purpose, including, but not limited to, the production of sodium hydroxide (NaOH), lithium hydroxide (LiOH), sodium metal, lithium metal, sodium or lithium alcoholates, sodium or lithium alkoxides, or to form any other suitable product in which an alkali ion plays a role in the product's production.

The described systems and methods may have several beneficial characteristics. In one example in which the alkali-ion-selective membrane 15 is water impermeable, a non-aqueous catholyte solution 40 can be used in the catholyte compartment 25 without being mixed with the aqueous anolyte solution 20. Accordingly, in such embodiments, the cell 10 can be used to carry out chemical reactions that may not be practical in a cell having a water-permeable membrane. Additionally, where the cell comprises an aqueous catholyte solution, the water-impermeable membrane in the described cell can allow the formation of a relatively concentrated alkali-hydroxide solution in the catholyte compartment, without allowing that solution to be diluted by water from the anolyte compartment.

In another example, because the cell 10 comprises a chlorine-gas-evolving anode 35 and because chlorine gas is rapidly removed from the cell, the pH of the anolyte compartment can remain relatively high (e.g., between about 4 and about 9) as the cell functions. Accordingly, the cell can use the acid-intolerant, alkali-ion-selective membrane 15 (e.g., a NaSICON-type or LiSICON-type material), with its beneficial characteristics (e.g., high ion conductivity, high selectivity for alkali ions, etc.) to produce chlorine gas from anolyte solution 20 comprising an alkali-chloride.

In still another example, some embodiments of the alkali-ion-selective membrane 15 are relatively pressure tolerant (e.g., allow for a difference in pressure between the anolyte compartment 20 and the catholyte compartment 25 to be as much as 250 pounds per square inch). Accordingly, in such embodiments, the cell 10 can operate while the anolyte compartment and catholyte compartment are subject to relatively different pressures. In other words, the cell can operate while the anolyte compartment is under a vacuum and the catholyte compartment is not.

While specific embodiments and examples of the present invention have been illustrated and described, numerous modifications come to mind without significantly departing from the spirit of the invention, and the scope of protection is only limited by the scope of the accompanying claims. 

1. An electrochemical cell comprising: an anolyte compartment; a gas-permeable, chlorine-gas-evolving anode disposed within the anolyte compartment; an aqueous alkali-chloride anolyte solution disposed in the anolyte compartment; a catholyte compartment comprising a cathode and a catholyte solution; an acid-intolerant, water-impermeable, alkali-ion-selective membrane separating the anolyte compartment from the catholyte compartment; and means for increasing a rate at which chlorine gas exits the anolyte compartment to inhibit a chemical reaction between the chlorine gas and the anolyte solution.
 2. The cell of claim 1, wherein the anolyte solution comprises a concentrated aqueous sodium chloride solution.
 3. The cell of claim 1, wherein the anolyte solution comprises a concentrated aqueous lithium chloride solution.
 4. The cell of claim 1, wherein the means for increasing the rate at which chlorine gas exits the anolyte compartment comprises a vacuum system.
 5. The cell of claim 1, wherein the means for increasing the rate at which chlorine gas exits the anolyte compartment comprises a heating system that provides heat to the anolyte solution.
 6. The cell of claim 1, wherein the anolyte compartment is disposed above the catholyte compartment in the cell, and wherein the anode is disposed in a substantially horizontal orientation within the anolyte compartment.
 7. The cell of claim 1, further comprising a hydrophobic membrane that is permeable to chlorine gas, wherein the hydrophobic membrane covers an opening of the anolyte compartment.
 8. The cell of claim 1, wherein the alkali-ion-selective membrane comprises a substance selected from a NaSICON-type material, sodium beta-alumina, a LiSICON-type material, lithium aluminum titanium phosphate (LATP), La_(x)Li_(y)TiO_(3-x) type perovskite, Li₂O—Al₂O₃—TiO₂—P₂O₅ glass, and Li₂S—P₂S₅ Thio-LiSICON type materials.
 9. A method for electrolyzing an alkali-chloride salt in an electrolytic cell comprising an acid-intolerant membrane, the method comprising: providing an electrochemical cell comprising: an anolyte compartment; a chlorine-gas-evolving anode disposed in the anolyte compartment; an aqueous alkali-chloride anolyte solution disposed in the anolyte compartment; a catholyte compartment comprising a cathode and a catholyte solution; and an acid-intolerant, water-impermeable, alkali-ion-selective membrane separating the anolyte compartment from the catholyte compartment; passing current between the anode and the cathode; and rapidly removing chlorine gas from the anolyte compartment to inhibit a chemical reaction between the chlorine gas and the anolyte solution.
 10. The method of claim 9, further comprising maintaining the anolyte solution at a temperature between about 40 and about 90 degrees Celsius.
 11. The method of claim 9, wherein the removing chlorine gas comprises using a vacuum to remove the chlorine gas from the anolyte compartment.
 12. The method of claim 9, wherein the anolyte solution comprises a concentrated aqueous sodium chloride solution.
 13. The method of claim 9, wherein the anolyte solution comprises a concentrated aqueous lithium chloride solution.
 14. The method of claim 9, wherein an alkali-chloride salt selected from sodium chloride and lithium chloride accounts for between about 10 and about 80%, by weight, of the alkali-chloride anolyte solution.
 15. The method of claim 9, wherein the alkali-ion-selective membrane is disposed horizontally within the cell, and wherein the anolyte compartment is disposed above the catholyte compartment.
 16. The method of claim 15, further comprising a hydrophobic membrane that is permeable to chlorine gas, wherein the hydrophobic membrane covers an opening of the anolyte compartment.
 17. A method for electrolyzing an alkali-chloride salt in an electrolytic cell comprising an acid-intolerant membrane, the method comprising: providing an electrochemical cell comprising: an anolyte compartment; a chlorine-gas-evolving anode disposed in a substantially horizontal orientation within the anolyte compartment; an aqueous anolyte solution disposed in the anolyte compartment, wherein the anolyte comprises an alkali-chloride salt selected from sodium chloride and lithium chloride; a catholyte compartment comprising a cathode and a catholyte solution; an alkali-ion-selective membrane separating the anolyte compartment from the catholyte compartment, wherein the alkali-ion-selective membrane comprises a substance selected from a NaSICON-type material, sodium beta-alumina, a LiSICON-type material, lithium aluminum titanium phosphate (LATP), La_(x)Li_(y)TiO_(3-x) type perovskite, Li₂O—Al₂O₃—TiO₂—P₂O₅ glass, and Li₂S—P₂S₅ Thio-LiSICON type materials; passing current between the anode and the cathode; and rapidly removing chlorine gas from the anolyte compartment through a process selected from heating the anolyte solution to a temperature between about 40 and about 90 degrees Celsius and applying a vacuum to the anolyte compartment.
 18. The method of claim 17, wherein the alkali-ion-selective membrane is disposed horizontally within the cell, and wherein the anolyte compartment is disposed above the catholyte compartment.
 19. The method of claim 18, further comprising a hydrophobic membrane that is permeable to chlorine gas, wherein the hydrophobic membrane covers an opening of the anolyte compartment.
 20. The method of claim 17, wherein the alkali-chloride salt accounts for between about 10 and about 40 percent, by weight, of the anolyte solution. 